Toxicity of Alexandrium lusitanicum to gastropod larvae is not caused by paralytic-shellfish-poisoning toxins

Abstract

Laboratory grazing experiments compared ingestion of two subclones of the dinoflagellate Alexandrium lusitanicum by gastropod veliger larvae (Nassarius sp.). While the two prey subclones originated from the same monoclonal isolate of A. lusitanicum, one possessed the ability to produce paralytic-shellfish-poisoning toxins (PSTs), while the other did not. Ingestion rates on the two Alexandrium subclones were not significantly different over a range of prey concentrations (approximately 100 - 660 cells ml^-1), indicating that PSTs did not serve as a grazing deterrent for these larvae. However, ingestion rates on both subclones were low at the higher prey concentrations tested. Mortality of the predators also increased linearly with concentration of either subclone. These observations indicated that both A. lusitanicum subclones produced an unknown substance that inhibited and killed the grazers. Veliger mortality was not induced by culture filtrates or lysates, suggesting either that the substance was either highly labile or that contact with intact cells was required. Because toxic algae can produce multiple bioactive substances, experimental demonstrations of alleopathic effects of toxic species should not be assigned to known toxins without supporting evidence. In addition, the results show that the effectiveness of algal grazing deterrents can increase with cell concentration, which may have implications for bloom dynamics.

Keywords: Alexandrium lusitanicum; Dinoflagellate; Harmful algal blooms (HABs); Nassarius; Paralytic shellfish poisoning (PSP); Red tide; grazing; saxitoxin; veliger.

Figure 1

Mean ingestion rates of veliger larvae fed PST-producing and non-PST-producing subclones of A. lusitanicum as a function of prey concentration. Further experimental details listed in Table 1. Error bars = 1 standard error. The solid line is a Gaussian function fitted to the mean values for both subclones (r² = 0.60).

Figure 2

Exponential mortality rate of veligers fed on either A. lusitanicum subclone as a function of prey concentration. Data are from the same experiments shown in Fig. 1 with the addition of data at zero (FSW) and 950 A. lusitanicum cells ml^-1.

Figure 3

C-based ingestion rates of Nassarius sp. veliger larvae on four species of algae. Ingestion rates of A. lusitanicum from this study (same data as Fig. 1, converted to C units), rates for the other algae taken from Pechenik and Fisher (1979). As in Fig. 1, the solid curve in the inset graph is a Gaussian function fitted to the mean ingestion rates on both A. lusitanicum subclones (r² = 0.60).